The goal of this research is to elucidate the cellular mechanisms whereby iron regulates folate metabolism and thereby facilitates cellular homeostasis. Folate is necessary for the synthesis of nucleotides and s-adenosylmethionine. These products are key metabolites for numerous reactions including DNA synthesis, DNA methylation, protein synthesis, and cell signaling. Although these products are required for all cells, folate metabolism is uniquely tailored in each cell type with respect to the source of single carbon units, the mechanisms of folate import, and the regulation of individual folate-related genes. Disruption of folate metabolism by vitamin deficiency, genetic predisposition or medical therapies leads to elevated serum homocysteine and increased uracil content in DNA; conditions that lead to tissue- and cell-specific pathologies. While we understand much about the individual biosynthetic reactions associated with folate metabolism, less characterized are the mechanisms that regulate folate metabolism, and thereby influence cellular processes. In this proposal, the biochemical mechanisms whereby iron metabolism regulates folate metabolism will be elucidated. We provide evidence that the enzyme serine hydroxymethyltransferase (cSHMT) displays robust nutrient regulation by iron metabolism and plays a key role in regulating the methylation of dUMP and homocysteine. The principle hypotheses to be tested are that: (1). cSHMT expression displays robust nutrient regulation by iron metabolism. (2). cellular iron regulates folate metabolism by altering cSHMT expression. (3). changes in cSHMT expression influence the expression of many genes in human MCF-7 cells. The long-term goals of this project are: (1). to define homeostatic mechanisms used by cells to regulate folate metabolism. (2). to elucidate the role(s) of cSHMT expression in the homeostatic control of folate metabolism.